Large mine laying submarines. Beneath the casing ran two rails which stretched the entire length of the casing. The mines were stowed on the rails until the time of laying the minefield. The casing had large stern doors which were opened when laying the mines and the mines were strewn through these stern doors.

HMS Porpoise (the first one of the class built) same specifications as above except;
Displacement: 1500 / 2053 BRT
Lenght: 288 feet.

Submarines of this class that have detailed patrol info added are shown in bold in the list below.